According to the conventional surgical operation for fixing and retrieving a deformed or injured vertebra, it is a common surgical practice that a foreign object such as a bone graft, or a filling material such as a hydroxylapitite, is implanted into the vertebra or between the vertebrae. However, the patient under treatment is rather susceptible to a severe pain caused by the implanted foreign object or filling material which is jutted out to touch the nerve or to injure the tissues contiguous to the vertebra under treatment. It is therefore necessary that a precautionary measure is taken to prevent the implanted foreign object or filling material from bulging out of the vertebra or from bulging out between the vertebrae. A bone plate is generally used as a means for preventing the implanted foreign object from jutting out; nevertheless the bone plate which covers the implanted foreign object can hinder the surgical operation. In addition, the implanted foreign object is even vulnerable to falling in view of the fact that the bone plate can not be fastened intimately with the vertebra intended to be fixed and that the bone screws can become loosened easily.